This invention relates to a gradient magnetic field coil which will be suitable for a magnetic resonance imaging apparatus. More specifically, in a gradient magnetic field coil to be used in combination with an open type superconducting magnet having a large aperture, this invention relates to a gradient magnetic field coil which can reduce an eddy current developing in a conductor in the proximity of the gradient magnetic field coil and can provide feeling of openness.
FIGS. 9 and 10 of the accompanying drawings show a first prior art example of a gradient magnetic field coil. These drawings depict the construction of a gradient magnetic field coil used for a horizontal field system superconducting magnet. The coil for the MR apparatus having the construction shown in FIGS. 9 and 10 is described, for example, in Japanese Patent Application laid-open No. JP-A-2-114942 filed in Japan as a Convention Priority application based on U.S. patent application Ser. No. 234,729 filed on Aug. 22, 1988. FIG. 9 is a sectional view of the superconducting magnet and FIG. 10 is an appearance view of the gradient magnetic field coil 2. Referring to FIG. 9, the superconducting magnet 1 generates a magnetic field in a horizontal direction (Z axis direction). Since the coil of the magnet uses a superconducting wire material, cooling must be made to a predetermined temperature (to a liquid helium temperature (4.2.degree. K) in the case of an alloy type superconductor and from a liquid nitrogen temperature (77.degree. K to 10.degree. K) in the case of an oxide type superconductor, for example). For this reason, the superconducting coil 41 is held inside a cooling container 3 comprising a vacuum vessel 3A and a cooling medium vessel 4 (a liquid helium vessel in FIG. 9).
In this prior art example, the gradient magnetic field coil 2 is constituted into a set of cylinders and generates gradient magnetic fields in X, Y and Z three directions orthogonally crossing one another in match with a three-dimensional space. To restrict an eddy current developing in a conductor (more concretely, the vacuum vessel 3A and a heat shield material (not shown) of the superconducting magnet) in the proximity of the gradient magnetic field coil 2, the gradient magnetic field coil 2 has been generally constituted in recent years by disposing coaxially a main coil 6 and a shield coil 7 in the case of a horizontal field system superconducting magnet. The main coil 6 primarily generates a predetermined gradient magnetic field in a homogeneous magnetic field area 5, and the shield coil 7 generates a magnetic field in the opposite direction to that of the main coil 6, so that the intensity of the magnetic field generated at the outside portion of the gradient magnetic field coil 2 can be reduced. Due to this operation, the occurrence of the eddy current in the proximate conductor can be effectively restricted. A subject is placed into the homogeneous magnetic field area 5.
In the construction shown in FIG. 9, however, the measurement space for accepting the subject for imaging is small as can be appreciated from the drawing. Further, because the subject is completely surrounded, the subject is obsessed with feeling of confinement and in some case, refuses to enter the space, and access to the subject from outside the apparatus is difficult.
FIGS. 11, 12A and 12B show the gradient magnetic field coil according to the second prior art example. This example uses an opposed type magnetic circuit using a permanent magnet in place of the superconducting magnet. FIG. 11 is an appearance view of the magnet as a whole and FIGS. 12A and 12B are an appearance view of the peripheral portion of the gradient magnetic field coil and a sectional view. As shown in FIG. 11, four sides are open in this example. The gradient magnetic field coil 2 used for this magnetic circuit is accommodated in a pole piece 9 constituting the magnetic circuit as shown in the drawing in most cases. This arrangement is important in order to limit the production cost of the magnetic circuit.
As described with reference to the horizontal field system superconducting magnet shown in FIGS. 9 and 10, therefore, the technology for restricting the eddy current cannot be applied to the permanent magnet system shown in FIGS. 11 and 12. For, in order to allow the shield coil to effectively operate, a certain distance must be secured between the shield coil and the main coil and in consequence, the main coil cannot be accommodated in the pole piece. As a result, the effective space for the subject is limited by the gap between the upper and lower gradient magnetic field coils opposing each other, becomes narrow and imparts the feeling of confinement to the subject.
The gradient magnetic field coil of the permanent magnet system shown in FIGS. 11, 12A and 12B is described, for example, in JP-A-63-65848.
In the case of the example shown in FIGS. 11, 12A and 12B, a technology of preventing the occurrence of the eddy current even when the gradient magnetic field is energized has been established by using a material having a high electric resistivity as the blank of the pole piece 9.
However, it is difficult to obtain a high magnetic field intensity in the case of the magnetic circuit using the permanent magnet 8 and about 0.3 Tesla is its upper limit. Since image quality of the magnetic resonance imaging apparatus greatly depends on the magnetic field intensity, it is preferred to obtain the highest possible magnetic field intensity to improve image quality.
FIGS. 13A and 14B show the gradient magnetic field coil according to the third prior art example. This example deals with the gradient magnetic field coil for use in a superconducting type opposed magnetic circuit, and is disclosed in U.S. Pat. No. 5,378,989.
FIG. 13A is an appearance view of a horizontal field type magnetic resonance imaging apparatus using the coil of the third prior art example, FIG. 13B is a sectional view on the plane taken along a line XIIB-XIIB' of FIG. 13A, and FIG. 14 is an appearance view of the gradient magnetic field coil. The example shown in these drawings uses a superconducting magnet 1 so as to obtain a high magnetic field intensity and openness, and has an open construction analogous to the permanent magnet system (a construction having a gap or a space at the center portion of a cylinder). In FIGS. 13A and 13B, the subject is loaded into an imaging area (homogeneous magnetic field area) at the center of the apparatus in the X or Z direction. Since the side surfaces of the imaging area 5 are open, the subject is released from the feeling of confinement. The operator of the apparatus or a doctor can get an easy access to the subject, and monitor of surgery is also possible.
Gradient magnetic field coils 2 comprising a cylindrical main coil 6 and a shield coil 7 are provided symmetrically with respect to the center plane 10 to the superconducting magnet 1 so as not to deteriorate openness of the magnetic circuit. Here, the main coil 6 has a cylindrical shape while the shield coil 7 has a cylindrical shape equipped with a flange round the outer periphery thereof as shown in FIG. 14. Since the shield coil 7 is so adapted as to cover the main coil 6, the occurrence of the eddy current in the conductor in the proximity of the gradient magnetic field coil 2 can be restricted.
To obtain openness, however, it is necessary to reduce the length in the direction of the depth (Z direction) of the apparatus as much as possible. To attain this object, the length of the gradient magnetic field coil 2 in the Z direction must be reduced, as well. To obtain a gradient magnetic field coil having sufficient performance (space linearity of the gradient magnetic field, generation efficiency of the gradient magnetic field and restriction of the eddy current), on the other hand, the length of the gradient magnetic field coil 2 in the Z direction must be elongated. Therefore, performance of the gradient magnetic field coil 2 must be limited if openness is to be improved.
U.S. Pat. No. 5,414,399 granted to Breneman et al. discloses an MRI apparatus using a superconducting magnet equipped with a pole piece for an opposed magnetic circuit.
As described above, it has been difficult to obtain a high performance gradient magnetic field coil suitable for a superconducting magnet for accomplishing a high magnetic field intensity and high openness and capable of satisfying required performance such as generation efficiency of a gradient magnetic field and restriction of an eddy current.